1. Field of the Invention
The present invention relates to a light-wavelength converting device using a Cerenkov radiation type phase matching for converting a laser light or the like into a light of a wavelength which is the half of that of the laser light.
2. Description of the Prior Art
As a light-wavelength converting device, there has been known a device which is formed in a shape of an optical fiber comprising a core of a non-linear optical crystal and a cladding or clad surrounding the core and which uses a Cerenkov radiation type phase matching. Such a light-wavelength converting device is also known as a second harmonics generator (hereinafter, abbreviated to "SHG") of the optical fiber type. According to the Cerenkov radiation method, the second harmonics in which the phase matching of the light is accomplished can be almost automatically generated, so that the SHG is applied to the light-wavelength converting apparatus or the like.
As shown in FIG. 1, an example of the light-wavelength converting apparatus comprises: a semiconductor laser 1; a coupling lens 2 for converging lights which are radiated from the semiconductor laser and for allowing the lights to enter an incident side edge surface of the SHG; an SHG 3 in which a core is formed by a non-linear optical crystal; and an axicon 4 for shaping a wave front of the second harmonics which have been converted by the SHG 3 and radiated and for obtaining the parallel light flux from the second harmonics. As mentioned above, there is a light source module including: the fiber type wavelength converting device; the coupling optical system for waveguiding the laser light radiated from the semiconductor laser, a YAG laser, or the like to the fiber type wavelength converting device; and the optical system for converting the wavelength converted second harmonics light into the parallel light.
FIG. 2 is a conceptional diagram of the operation of the SHG mentioned above. The SHG comprises: a cylindrical core 10 and a cylindrical clad layer 20 which concentrically surrounds the core 10.
In FIG. 2, when a fundamental wave mode is index N(.omega.) in the direction from the left to the right in the diagram, a non-linear polarization wave which generates the second harmonics is also propagated at the same phase velocity C/N (.omega.) (C: light velocity). It is assumed that the non-linear polarization wave had generated the second harmonics at a point A in the diagram in the direction of a Cerenkov radiation angle .theta. from the waveguide direction and again has generated the second harmonics at a point B in the .theta. direction in a manner similar to the above after the elapse of a unit time. If the second harmonics generated at the A point are propagated, for instance, in the clad layer 20 and reach a point C after the elapse of a unit time and the Cerenkov radiation angle .theta. is equal to an angle at which the line segments AC and BC perpendicularly cross, the wave front of the second harmonics in which the non-linear polarization wave has been generated in a range between the points A and B is set to the line segment BC, so that the coherent second harmonics have eventually produced.
The sound harmonics generated as mentioned above are propagated as a clad mode such that the total reflection is repeated at the boundary between the clad layer 20 and the air. As shown in FIG. 3, the second harmonics are emitted like a circular cone from the outgoing edge surface of the SHG in the direction which is determined by the Cerenkov radiation angle .theta.. The equiphase front of the outgoing wave front of the second harmonics emitted as mentioned above has a circular cone shape using a center shaft of the fiber axis.
As the above SHG, there has been proposed a fiber type wavelength converting device of the Cerenkov radiation type which is formed by a method whereby an organic non-linear material fused into a glass capillary having an outer diameter of about 1 to mm, an inner diameter of 1 to .mu.m, and a length of a few mm to ten and a few mm is sucked by the capillary phenomenon and, after that, it is crystallized. As a core material, an organic non-linear optical material, MNA, DAN, NPP, PNP, DMNP, MMONS, etc. can be used. In the case of using such an SHG to a light-wavelength converting apparatus such as a light source module mentioned above or the like, a simple coating is performed to an incident side edge surface 21 and an outgoing side edge surface 22 of the SHG, thereby preventing a deterioration by sublimation and moisture absorption of the organic non-linear material in the core.
In the case where thicknesses of coating films of those edge surfaces lie within a range about from hundreds .ANG. to thousands .ANG., however, the sublimation and the moisture absorption of the organic non-linear material cannot be sufficiently prevented. Further, a deterioration of the incident light coupling efficiency due to the deposition of dusts onto the incident edge surface cannot be suppressed. It is necessary to increase the fiber length of the SHG in order to improve the wavelength conversion efficiency. The effective operation length of the SHG is equal to a length which is required until the second harmonics which are generated are reflected by the interface between the glass of the clad and the air and are again returned to the core. Consequently, the operation length can be increased only by setting the outer diameter of the SHG to a large value. Since it is difficult to manufacture a thick capillary serving as a clad, it is not easy to form a high efficient wavelength converting device.